Answer: C
Explanation: The assumption that pairing of electrons reduces orbital stability is not part of Bohr's theory.
Bohr's theory of atomic structure mainly focuses on the behavior of electrons in the atom and makes several key assumptions:
a) Energy is not dissipated when electrons move in an orbital (postulate of quantization)
b) Electrostatic force balances force tending to throw electron out of orbital (Coulomb's law)
d) Electrons move from one orbital to another by either absorbing or radiating energy (postulate of transitions)
Therefore, the correct answer is option (c) Pairing of electrons reduces orbital stability.